Electrochemical anodization of target substrates is in use on various scenes. Such anodization includes treatment in which a polycrystalline silicon layer is made porous. The outline thereof is such that the target substrate having the polycrystalline silicon layer formed on the surface thereof is electrically connected to a positive potential pole of a power supply via a conductor and immersed in a hydrofluoric acid solution dissolved in a solvent (for example, ethyl alcohol). An electrode made of, for example, platinum is immersed in the hydrofluoric acid solution, in other words, in a chemical, and is electrically connected to a negative potential pole of the above-mentioned power supply. Further, the polycrystalline silicon layer on the target substrate immersed in the chemical is irradiated with light by a lamp.
This causes the polycrystalline silicon layer to partly melt in the hydrofluoric acid solution. Pores are formed where the polycrystalline silicon layer has been melted, so that the silicon layer is turned into a porous structure. The photoirradiation by the lamp is intended for producing holes necessary for the reaction of the above-mentioned melting and pore formation in the polycrystalline silicon layer. For reference, such reaction in the polycrystalline silicon layer in the anodization is explained, for example, as follows.Si+2HF+(2−n)e+→SiF2+2H++ne−SiF2+2HF→SiF4+H2SiF4+2HF→H2SiF6Here, e+ is a hole and e− is an electron. Therefore, this reaction requires holes as a precondition and is different from simple electrolytic polishing.
The porous silicon thus produced is made suitable as a highly efficient field emission electron source by further forming a silicon oxide layer on a nano-level surface thereof, which is disclosed in, for example, Japanese Patent Laid-open No. 2000-164115, Japanese Patent Laid-open No. 2000-100316, and so on. The use of such porous silicon as the field emission electron source has been drawing attention as opening a door to realizing a new flat display device.
In the anodization as described above, a value of an electric current flowing to a cathode electrode through a chemical from the target substrate is proportional to the area of the target substrate (the area of a treatment part). This is because the electric current promotes the reaction, and the reaction is caused uniformly on respective portions in the surface of the target substrate. Therefore, when the target substrate is intended for use in a large display device and thus has a large area, the value of the electric current necessary for the treatment remarkably increases. For example, assuming that a target substrate of 200 mm square requires a treatment current of about 5 A, an electric current of 100 A, which is 25 times as large, is required for a target substrate of 1000 mm square. Note that the area substantially equal to 1000 mm square is a commonly conceivable value from the viewpoint of the future trend of large display devices.
An apparatus to supply such a large electric current naturally has a large power supply section or the like, which makes the apparatus expensive. Further, the area of the photoirradiation by a light source is increased, and the shape of the cathode electrode becomes large, which also causes the cost increase of the apparatus. This is also reflected in the manufacturing cost of substrates manufactured by the apparatus.
Further, from a different viewpoint, there also exists a problem of deterioration in uniformity of the anodization in the surface of the target substrate because the increase in the area of the photoirradiation by the light source makes it difficult to achieve a uniform amount of photoirradiation of the target substrate and because the increase in the size of the cathode electrode makes it difficult to ensure uniformity of an electric field formed between the cathode electrode and the target substrate. This is problematic in ensuring quality of manufactured substrates.